David Hildenbrand <da...@redhat.com> writes: > On 09.06.23 08:08, Aneesh Kumar K.V wrote: >> Certain devices can possess non-standard memory capacities, not constrained >> to multiples of 1GB. Provide a kernel parameter so that we can map the >> device memory completely on memory hotplug. > > So, the unfortunate thing is that these devices would have worked out of > the box before the memory block size was increased from 256 MiB to 1 GiB > in these setups. Now, one has to fine-tune the memory block size. The > only other arch that I know, which supports setting the memory block > size, is x86 for special (large) UV systems -- and at least in the past > 128 MiB vs. 2 GiB memory blocks made a performance difference during > boot (maybe no longer today, who knows). > > > Obviously, less tunable and getting stuff simply working out of the box > is preferable. > > Two questions: > > 1) Isn't there a way to improve auto-detection to fallback to 256 MiB in > these setups, to avoid specifying these parameters? > > 2) Is the 256 MiB -> 1 GiB memory block size switch really worth it? On > x86-64, experiments (with direct map fragmentation) showed that the > effective performance boost is pretty insignificant, so I wonder how big > the 1 GiB direct map performance improvement is.
The other issue is simply the number of sysfs entries. With 64TB of memory and a 256MB block size you end up with ~250,000 directories in /sys/devices/system/memory. cheers